By ob­serv­ing wild orang­u­tans, re­search­ers have con­clud­ed that our up­right walk­ing style may have first aris­en in rel­a­tively an­cient, tree-dwelling apes.

The the­o­ry chal­lenges cur­rent think­ing, which sug­gests two-legged walk­ing emerged in more re­cent hu­man an­ces­tors that had al­ready de­scended to flat lands. The new pro­po­sal ap­pears in the June 1 is­sue of the re­search jour­nal Sci­ence.

Up­right walk­ing, or bi­pe­dal­ism, has long been con­sid­ered a de­fin­ing fea­ture of hu­mans and our clos­est an­ces­tors. One of the lead­ing ex­plana­t­ions, known as the sa­van­nah hy­poth­e­sis, sug­gests that the an­ces­tors of chimps, go­ril­las and hu­mans de­scended from the trees and be­gan walk­ing on on all fours.

Over time, this four-leg­ged gait would have evolved in­to the “knuckle-walk­ing” that chimps and go­ril­las still use to­day; up­right, two-legged walk­ing would have been the next stage.

Pa­le­on­tol­o­gists have con­ven­tion­ally used signs of bi­pe­dal­ism as key cri­te­ria for dis­tin­guish­ing early hu­man, or “ho­min­in,” fos­sils from those of oth­er apes.

But this dis­tinc­tion is com­pli­cat­ed by re­cent fos­sil ev­i­dence that some early ho­minins, in­clud­ing the crea­ture dubbed “Lucy” (Aus­tra­lo­pith­e­cus afaren­sis), lived in wood­lands. Still ear­li­er forms seem to have lived in the for­est can­o­py and moved on two legs.

“Our find­ings blur the pic­ture even fur­ther,” said Rob­in Cromp­ton of the Un­ivers­ity of Liv­er­pool, U.K., one of the study’s au­thors. “If we’re right, it means you can’t rely on bi­pe­dal­ism to tell wheth­er you’re look­ing at a hu­man or oth­er ape an­ces­tor. It’s been get­ting more and more dif­fi­cult for us to say what’s a hu­man and what’s an ape, and our work makes that much more the case.”

Cromp­ton and col­leagues at the Un­ivers­ity of Bir­ming­ham in Bir­ming­ham, U.K., drew their con­clu­sions by watch­ing wild orang­u­tans in Su­ma­tra, In­do­ne­sia. Orangutans spend al­most their whole lives in trees, mak­ing them use­ful mod­els for how our an­ces­tors moved around sev­er­al mil­lion years ago, Cromp­ton and col­leagues said.

To col­lect the da­ta, the Un­ivers­ity of Bir­ming­ham’s Su­san­nah Thorpe spent a year in the Su­ma­tran rainfor­est re­cord­ing vir­tu­ally eve­ry move the orang­u­tans made. Then, she and her col­leagues used these ob­serva­t­ions to test the hy­poth­e­sis that bi­pe­dal­ism would have ben­e­fit­ed tree-dwelling ape an­ces­tors.

Be­cause these an­ces­tors were probably fruit-eaters, as orang­u­tans are, they would have needed a way to nav­i­gate the thin, flex­i­ble branches at the tree’s pe­riph­ery, where the fruit typ­ic­ally is. Mov­ing on two legs and us­ing their arms pri­marily for bal­ance, or “hand-as­sisted bi­pe­dal­ism,” may have helped them trav­el on these branches, the re­search­ers said.

They an­a­lyzed nearly 3,000 ex­am­ples of orang­u­tan move­ment, and found that the orang­u­tans were more likely to use hand-assisted bi­pe­dal­ism when they were on the thinnest branches. When bi­pe­dal, the an­i­mals al­so tended to grip mul­ti­ple branches with their long toes.

On me­di­um-sized branches, the orang­u­tans used their arms more to sup­port their weight, chang­ing their mov­ing style to in­cor­po­rate hang­ing. They only tended to walk on all fours when nav­i­gat­ing the larg­est branches, the re­search­ers found.

Hand-assisted bi­pe­dal­ism may have of­fered sev­er­al ad­van­tages that al­lowed our tree-dwel­ling an­ces­tors to ven­ture on­to thin branches, Cromp­ton and col­leagues argued. The ani­mals could have gripped mul­ti­ple branches with their toes and dis­trib­ut­ed their cen­ter of gra­vity more ef­fec­tive­ly, while keep­ing one or both of their long arms free to reach for fruits and oth­er sup­ports.

Orangutans al­so keep their legs straight while stand­ing on bend­ing branches, the au­thors re­ported. The ben­e­fit of the straight legs is un­clear, but when hu­mans run on springy sur­faces, we al­so keep our weight-bearing legs straighter, so this may have an en­er­gy-related ad­van­tage.

“Our re­sults sug­gest that bi­pe­dal­ism is used to nav­i­gate the small­est branches where the tasti­est fruits are, and al­so to reach fur­ther to help cross gaps be­tween trees,” said Thorpe.

The au­thors pro­pose an ev­o­lu­tion­ary sce­nar­i­o that be­gins as oth­er re­search­ers have en­vi­sioned. Some­where to­ward the end of the Mi­o­cene era, 24 to 5 mil­lion years ago, cli­mate in East and Cen­tral Af­ri­ca be­came al­ter­nately wet­ter and dri­er, and the rainfor­est in­creas­ingly patchy. Apes liv­ing in the for­est can­o­py would have be­gun to en­coun­ter gaps be­tween trees that they couldn’t cross through the high can­o­py.

Hu­man an­ces­tors would thus have aban­doned the high can­o­py for the for­est floor, where they re­mained bi­pe­dal and be­gan eat­ing food from the ground or smaller trees. The an­ces­tors of chimps and go­ril­las, on the oth­er hand, be­came more spe­cial­ized for ver­ti­cal climb­ing be­tween the high can­o­py and the ground. They thus would have de­vel­oped knuckle-walk­ing for cross­ing from one tree to anoth­er on the ground.

The upright style, called “ar­bor­e­al bi­pe­dal­ism, had very strong adap­tive ben­e­fits. So, we don’t need to ex­plain how our an­ces­tors could have gone from be­ing quad­ru­pe­dal to be­ing bi­pe­dal,” Thorpe said. Ob­serva­t­ions of orang­u­tan move­ment should be use­ful for con­serva­t­ion ef­forts, ac­cord­ing to Thorpe. These an­i­mals are se­ri­ously en­dan­gered, mainly be­cause of hab­i­tat de­struc­tion.

“If you can un­der­stand how they cross gaps in the for­est, you can learn about ef­fects that liv­ing in logged or de­grad­ed hab­i­tat would have on their lo­co­mo­tion. These could af­fect en­er­gy lev­els, for ex­am­ple, if they have to go to the ground, which is in­credibly risky be­cause the Su­ma­tran ti­ger is down there lick­ing its lips. The Su­ma­tran orang­u­tan popula­t­ion is pre­dicted to be ex­tinct in the next dec­ade if hab­i­tat de­grada­t­ion con­tin­ues. Our re­search fur­ther high­lights the need for pro­tect­ing these an­i­mals,” she said.

Courtesy American Association for the Advancement of Science and World Science staff